PROJECT SUMMARY Cells dynamically assemble a diverse set of actin-based structures to perform essential processes including motility, division and signaling. One class of actin structures consists of networks of actin filaments that are bundled together into parallel or anti-parallel architectures. These bundled actin structures include contractile rings, actin cables, filopodia, and the stress fibers that promote focal adhesion maturation and nuclear positioning. The actin filaments that comprise these structures are primarily polymerized by formins, a family of proteins that nucleate and direct the elongation of unbranched actin filaments. Mammals express 15 formin isoforms, each of which possesses unique actin assembly properties and plays a specific role in cells. Although mutations in formin genes are associated with a number of human diseases including preleukemic disorders and cancer, it remains unknown how the polymerization activity of each formin isoform is tuned for the assembly of a specific bundled actin structure and participation in a specific cellular process. To bridge this gap in understanding, we must establish how formins function in the context of the bundled actin structures that exist in cells. We will address this question by characterizing the mechanism of formin-mediated assembly of bundled actin structures. Our central hypothesis is that incorporation of formin-bound actin filaments into bundled structures modulates polymerization by exposing formins to force, filament bundling and filament severing. We will use a combination of biophysical and cell biological approaches to test this hypothesis by pursuing three specific aims: (1) To elucidate the effects of force on the mechanism of formin-mediated filament elongation, (2) to investigate the relationship between filament bundling and formin polymerization activity, and (3) to evaluate the contribution of filament turnover to bundled actin structure assembly. By establishing the mechanism of formin-directed bundled actin assembly, we will gain fundamental insights into the large number of cellular processes regulated by formins. This work will also provide a molecular basis for understanding how formins contribute to healthy cellular proliferation and normal development.